Issue link: https://iconnect007.uberflip.com/i/1398328
AUGUST 2021 I SMT007 MAGAZINE 75 activity of a flux. e wetting results of Alpha's PV-21 are given below in Figure 4. e equilib- rium wetting force figure shows, that the flux has wetting time of 0.43 sec and wetting force of 7 gF. is mean PV-21 and similar fluxes can be used in a high throughput environment as they can be activated in a shorter time. e equilibrium wetting force is a function of both the solid-liquid and the solid-vapor interface. Both interfaces may depend on the presence of the flux. e solid-vapor interface will be influ- enced by the presence of the flux. In addition, different fluxes will remove surface oxides from the solid with different efficiencies. Usually, the rate of wetting increases as the acid con- tent of the flux increases. e rate of wetting is dependent on the degree of oxide removal, which is a function of acid concentration and temperature. is indicates that the flux with better wetting time effectively removes surface oxides within the first [18,19] . However, fluxes with classification L are preferred as higher acid number or halogenated may cause corro- sion issues [19] . Shelf life of flux is another important fac- tor. Typically, fluxes are stable for six months. In today's environment, longer shelf-life fluxes are preferred. A good flux should have [19] : • Excellent wetting and spread • Higher acid number (15-21 mg/g of KOH) • Longer shelf life • No harmful or corrosive residues • Rapid oxide removal characteristics • Reliability as evidenced by Cu mirror and corrosion test per IPC standard • No interaction with EVA • Class "L" classification and excellent wettability In addition, since most of the MBB tabbing and stringing operation is done by dip coating method, higher solid content (1.6-3 %) flux is preferred. Jae Hun Kim et al [17] have reported that flux including high solid material can affect to solder behavior on Ag electrode in MBB interconnection. Solder tends to migrate towards the edge of the wire during soldering, thus leading to lower growth of IMC layer. is thin and uniform IMC layer helps in increasing fill factor. ey proved that in a soldering pro- cess, the amount of solid material in flux and the solder thickness, determines characteris- tics of solder bond. e right balance of these along with contamination-free operation is a key to achieve high throughput manufacturing as well as reliable solder joints. Process Conditions Effect of solder time and temperature e MBB soldering operation is preferably done at higher temperature for better thermal contact and achieving high throughput. is has one downside, due to the different ther- mal expansion of the copper and the silicon elements; solder joints can form microcracks, which might not be detected during the man- Figure 3: a) Image and b) X-ray image sowing copper wire with solder bump. No voids are seen in X-ray image.